CHEVY LOSS OF POWER

2000 Chevrolet Tracker

2000 Chevy Tracker 6 cyl Four Wheel Drive Automatic

I was wondering if you can tell me why my car is shutting off when I am idle or sometimes if I start the car to leave it comes on but stops as soon as I get to a stop sign or in traffic. I put it on a diagnostic machine and it said the catalytic converter needed to be changed, so I did just that but I am still having the same issues.